1. Field of the Invention
The invention relates to anodes intended for the production of aluminium by electrolysis. It particularly relates to the so-called “inert” or “non consumable” anodes and their preparation and use.
2. Description of the Related Art
Metal aluminium is produced industrially by fused bath electrolysis, namely by electrolytic reduction of alumina in solution in a molten cryolite bath called an electrolyte bath, particularly using the well-known Hall-Héroult process. The electrolytic reduction is made in electrolytic cells comprising an electrolytic pot provided with carbon cathode elements and one or several anodes. The electrolyte bath is contained in the pot and the anodes are partially immersed in the electrolyte bath. The electrolytic current is used to maintain the electrolyte bath at the temperature required by the Joule effect. The electrolytic cell is fed regularly with alumina so as to compensate for alumina consumption caused by the electrolysis reactions.
In the standard technology, anodes are made of a carbonaceous material and the electrolysis is done at a temperature typically of the order of 950° C. Since the anodes made of a carbonaceous material are progressively consumed during the electrolysis, the height of the part of the anodes that is immersed in the bath needs to be frequently adjusted and action needs to be taken on the cell to replace the anodes. Moreover, degradation of anodes produces carbon dioxide (more than one tonne of CO2 per tonne of aluminium produced) which contributes to the greenhouse effect.
At the present time, research is being carried out on the design of so-called inert or non-consumable anodes with a very low wear rate, namely preferably less than 1 cm/year, in order to obtain lives longer than one year and to produce a metal with commercial purity.
In particular, it has been proposed to use composite materials with a ceramic matrix containing one or several metallic phases as electrode materials. In particular, these metallic phases can improve the thermomechanical properties of electrodes that are subjected to high thermal stresses that could deteriorate them. This type of composite materials, that contain at least one “ceramic” phase and at least one metallic phase, is known under the term “cermet”.
Studies have been carried out particularly on cermets for which the ceramic phase is a mixed phase of nickel oxide (NiO) and nickel ferrite (NiFe2O4), and for which the metallic phase for example contains iron, nickel or copper; for example, see U.S. Pat. Nos. 4,454,015, 4,455,211 and 4,582,585. Several recent patents relate to NixFe3-xO4/Ni1-yFeyO/Cu type cermets, in other word based on nickel ferrite and nickel oxide, the metallic phase being mainly copper.
Cermets of this type are typically produced using a procedure with four main steps:                a mixture of oxide powers (for example NiFe2O4 and NiO, or Fe2O3 and NiO), and metallic copper,        the addition of an organic binder to the previous powder mixture, to obtain a “bonded” powder,        pressing (uniaxially or isostatically) of the bonded powder to obtain a coherent “green” solid, with the shape defined by the geometry of the pressing mould,        a heat treatment of the green solid under controlled atmosphere at a temperature of about 1300° C., to decompose the binder and sinter the powder.        
French application FR 03-03045 by Aluminium Pechiney describes such a production process.
The microstructure of the cermet obtained then includes spinel ferrite grains, nickel oxide grains and metallic particles with an average size typically more than 10 μm in the case of the copper. Some copper is usually bled out at the surface of the cermet. Metallic drops with a diameter between about 100 microns and several millimetres can thus be observed. It is usually necessary to eliminate this roughness by appropriate chemical or mechanical treatments before the cermet is used in an electrolytic cell. These treatments increase manufacturing costs and are usually difficult to implement.
Furthermore, the process for making such cermets requires strict control over the sintering atmosphere to avoid oxidising the copper. The role of the organic binder is to facilitate shaping and to make sure that the “green” material is cohesive, and it must not react with oxides or metallic phases. Moreover, during the heat treatment, decomposition of the binder (the so-called “debinding” step) under a neutral or slightly oxidising atmosphere leads in particular to the formation of reducing unsaturated carbonaceous species that could modify the chemical composition or the microstructure of the cermet. Therefore, control over the debinding step is very difficult.
The applicant searched for solutions that could reduce or even prevent these disadvantages.